1. Field of the Invention
The present invention relates to a method for producing an aromatic polycarbonate. More particularly, the present invention is concerned with a method for producing an aromatic polycarbonate, comprising: feeding, into a polymerizer apparatus, at least one polymerizable material selected from the group consisting of a molten monomer mixture of an aromatic dihydroxy compound and a diaryl carbonate, and a molten prepolymer obtained by a process comprising reacting an aromatic dihydroxy compound with a diaryl carbonate, the polymerizer apparatus including a guide-wetting fall polymerization reaction zone which has at least one guide securely held therein and extending downwardly therethrough, and allowing the polymerizable material to fall along and in contact with the at least one guide through the guide-wetting fall polymerization reaction zone, to effect a guide-wetting fall polymerization of the polymerizable material, thereby obtaining a polymer, wherein the guide is a perforated wall-surface guide. The present invention is also concerned with a polymerizer apparatus for practicing the above-mentioned method.
The method of the present invention for producing an aromatic polycarbonate is free from the problems accompanying the conventional method: such as the problem that phenol cannot be removed efficiently; the problem that a very large motive power for agitation is needed; the problem that the molecular chain of a polymer being formed is broken by a large shearing force due to the very large motive power for agitation, resulting not only in a reduced rate of increasing molecular weight but also in a discoloration of the polymer. Also, there is the problem that the polymer is likely to suffer thermal decomposition due to a long-term thermal history, thereby inevitably causing the polymer being produced to contain thermal decomposition products, and there is the problem that the volumetric efficiency of a polymerizer is extremely low.
By the method of the present invention, an aromatic polycarbonate having a desired constant molecular weight can be stably produced at a high polymerization rate, without discoloration with respect to the polymer or generation of foreign matter. Therefore, the method of the present invention is advantageous from a commercial viewpoint.
2. Prior Art
In recent years, aromatic polycarbonates have been widely used in various fields as engineering plastics having excellent heat resistance, impact resistance and transparency. With respect to methods for producing aromatic polycarbonates, various studies have heretofore been made. Of the methods studied, a process utilizing an interfacial polycondensation between an aromatic dihydroxy compound, such as 2,2-bis(4-hydroxyphenyl)propane (hereinafter, frequently referred to as xe2x80x9cbisphenol Axe2x80x9d), and phosgene has been commercially practiced.
However, the interfacial polycondensation process has problems. First, it is necessary to use phosgene, which is poisonous. Second, the reaction apparatus is likely to be corroded with chlorine-containing compounds, such as hydrogen chloride and sodium chloride (which are by-produced) and methylene chloride (which is used as a solvent in a large quantity). Third, difficulties are encountered in separating and removing impurities, such as sodium chloride, and the residual methylene chloride, which adversely affect properties of a produced polymer.
With respect to a method for producing an aromatic polycarbonate from an aromatic dihydroxy compound and a diaryl carbonate, a transesterification process has conventionally been known, in which a polycarbonate is produced by performing an ester exchange reaction between bisphenol A and diphenyl carbonate in the molten state, while removing a by-produced phenolic compound (such as phenol). Unlike the interfacial polycondensation process, the transesterification process has an advantage in that a solvent need not be used. However, the transesterification process has a serious problem, namely; since the viscosity of a polymer being formed increases during the progress of the polymerization reaction, it becomes difficult to remove by-produced phenol from the polymerization reaction system efficiently, thus making it difficult to achieve a high degree of polymerization with respect to the polycarbonate produced.
Various polymerizers have been conventionally used in producing aromatic polycarbonates by the transesterification process. A vertical agitation type polymerizer vessel equipped with an agitator is widely used. The vertical agitation type polymerizer vessel equipped with an agitator is advantageous in that it exhibits high volumetric efficiency and has a simple construction, so that polymerization on a small scale can be efficiently carried out. However, the vertical agitation type polymerizer vessel has a problem in that, as mentioned above, the by-produced phenol becomes difficult to remove from the polymerization reaction system efficiently in the production of aromatic polycarbonates on a commercial scale, so that the polymerization rate becomes extremely low.
Specifically, a large-scale vertical agitation type polymerizer vessel generally has a greater ratio of liquid volume to vaporization area than a small-scale one. In other words, the depth of a reaction mixture in the polymerizer is large and, hence, the pressure in the lower part of the agitation vessel is large. In such a case, even if the degree of vacuum of the polymerization reaction zone is raised in order to achieve a high degree of polymerization in the lower part of the agitation vessel, the polymerization proceeds under virtually high pressure due to the weight of the reaction mixture, so that phenol and the like cannot be efficiently removed.
To solve the above-mentioned problem, various attempts have been made to remove phenol and the like from high viscosity polymer being formed. For example, Examined Japanese Patent Application Publication No. 50-19600 (corresponding to GB-1 007 302) discloses the use of a screw type polymerizer having a vent. Examined Japanese Patent Application Publication No. 52-36159 discloses the use of an intermeshing twin-screw extruder. Examined Japanese Patent Application Publication No. 53-5718 (corresponding to U.S. Pat. No. 3,888,826) describes a thin film evaporation type reactor, such as a screw evaporator and a centrifugal film evaporator. Further, Unexamined Japanese Patent Application Laid-Open Specification No. 2-153923 discloses a method in which a combination of a centrifugal film evaporator and a horizontal agitation type polymerizer vessel is used.
Of these polymerizers, horizontal polymerizers, such as a screw evaporator and a horizontal agitation type polymerizer vessel, are intended to increase, by rotary agitation, the surface renewal of polymer (being formed) to a level as high as possible in an attempt to remove phenol and the like efficiently. For example, Examined Japanese Patent Application Publication No. 50-19600 describes that xe2x80x9cA relatively large, continuously renewing interface is formed between the liquid reaction system and the ambient gas or vapor, so that a volatile reaction product formed in the liquid reaction system is extremely smoothly removed.xe2x80x9d (see page 1, right-hand column, lines 19 to 22 of the above patent document). That is, the above patent document suggests that phenol and the like can be efficiently removed by the renewal of gas-liquid interface. Further, in Examined Japanese Patent Application Publication No. 52-36159, surface renewal effect xe2x80x9cJxe2x80x9d is defined as a function of the screw revolution rate, the screw surface area in the reaction zone, the total screw pitch number in the reaction zone, the feed amount of raw material and the effective volume per screw pitch in the reaction zone, and it is described that it is important that the value of the surface renewal effect be in a specific range.
However, these horizontal polymerizers need rotary agitation force provided by, for example, a screw or an agitator, for increasing the surface renewal. It should be noted that the viscosity of an aromatic polycarbonate being formed increases extremely in accordance with the increase in the molecular weight thereof during the polymerization reaction, so that an extremely large agitation force becomes necessary. In addition, when a large agitation force is exerted on a polymer having a high viscosity, the polymer sustains a large shearing force and, hence, breakage of the molecular chain occurs, so that the increasing rate of the molecular weight becomes low, making it impossible to obtain an aromatic polycarbonate having a high molecular weight. Further, when an aromatic polycarbonate sustains a large shearing force, a discoloration of the polycarbonate occurs, so that the quality of the aromatic polycarbonate is seriously adversely affected. Furthermore, when the production of an aromatic polycarbonate by using an agitation type polymerizer is performed on a commercial scale, the size of the agitation type polymerizer is inevitably limited. This is due to the fact that when there is an increase in the size of the agitation type polymerizer, it is necessary to increase the strength of the agitator and the motive power for agitation; however, there are limits to this increase in strength and motive power. Therefore, with the use of an agitation type polymerizer, the production amount of the aromatic polycarbonate cannot be easily increased. That is, agitation type polymerizers also have a problem in that a scale-up of the production of an aromatic polycarbonate is difficult.
With respect to centrifugal film evaporators, Unexamined Japanese Patent Application Laid-Open Specification No. 2-153923 has a description to the effect that, by using a centrifugal film evaporator as a polycondensation reactor in the final stage of a transesterification reaction, the evaporation surface area of the liquid reaction system per unit weight of the liquid reaction system can be increased, thereby enabling a decrease in the residence time of the liquid reaction system in the reactor. However, the above patent document also points out the following problems. That is, when a centrifugal film evaporator is used, a part of the polymer being formed sticks to the surfaces of the driving shaft, the blade, the bearing for the driving shaft, and the like, and is exposed to a thermal environment for a long period of time, so that the part of the polymer sticking to the surfaces is decomposed to form a black decomposition product, and the black decomposition product undesirably enters the polymer being produced. In order to obviate this problem, the above patent document discloses a method in which a centrifugal film evaporator is used in the middle stage of the transesterification reaction, but not in the final stage of the reaction. However, in this method, a film of polymer is formed only on the inner wall surface of the evaporator and, hence, the volumetric efficiency of the evaporator as a polymerizer is extremely low, so that a satisfactory reaction time cannot be obtained without using a reactor which is too large. Thus, the centrifugal film evaporator cannot be suitably used on a commercial scale.
With respect to a polymerizer having no rotary agitator, Examined Japanese Patent Application Publication No. 48-8355 discloses a polymerizer for producing a polyester or a polyamide, wherein the polymerizer comprises a cylindrical reactor casing and a porous body disposed therein and extending substantially vertically, wherein the cylindrical reactor casing is equipped with a heater, a polymerizable material distribution device positioned above the porous body, a polymerization product discharging device positioned below the porous body, and a device for removing gas generated from a reaction mixture produced in the polymerizer, and wherein the porous body has a void ratio distribution which increases from the upper end thereof to the lower end thereof. However, in this document, there is no disclosure about a production of an aromatic polycarbonate, that is, there is no disclosure as to how to make it possible to stably produce an aromatic polycarbonate having a desired constant molecular weight at a high polymerization rate, without discoloration with respect to the polymer or generation of foreign matter.
As described hereinabove, in the production of an aromatic polycarbonate by the transesterification process (which is free from the problem that impurities and residual methylene chloride are contained in the produced polycarbonate and these unfavorable substances cannot be completely removed therefrom), when the transesterification process is performed by the conventional production methods using a vertical agitation type polymerizer, a horizontal agitation type polymerizer, a centrifugal film evaporator, or the like, various problems arise. For example, phenol cannot be removed efficiently; a very large motive power for agitation is needed; the molecular chain of a polymer being formed is broken by the shearing force due to the very large motive power for agitation, resulting in a lowering of the increasing rate of the molecular weight and in a discoloration of the polymer; a thermal decomposition product of a polymer is generated due to a longterm thermal history, which undesirably enters the polymer being produced; the volumetric efficiency of a polymerizer is extremely low; and a scale-up of the production of an aromatic polycarbonate is difficult.
As a solution to these problems, in U.S. Pat. No. 5,589,564, the present inventors disclosed that a high quality aromatic polycarbonate can be produced at a high polymerization rate by a method in which a polymerizable material is allowed to fall along and in contact with a wire to thereby effect a polymerization reaction. However, it have been desired to develop a method which enables an aromatic polycarbonate having a desired constant molecular weight to be stably produced at a higher polymerization rate.
As apparent from the above, a commercially advantageous method for producing an aromatic polycarbonate has been desired, wherein the method is free from the above-mentioned problems accompanying the conventional methods and enables an aromatic polycarbonate having a desired constant molecular weight to be stably produced at a high polymerization rate, without discoloration a with respect to the polymer or generation of foreign matter.
In this situation, the present inventors have conducted extensive and intensive studies with a view toward solving the above-mentioned problems. As a result, it has unexpectedly been found that the above-mentioned objective can be attained by a method which comprises: feeding, into a polymerizer apparatus, at least one polymerizable material selected from the group consisting of a molten monomer mixture of an aromatic dihydroxy compound and a diaryl carbonate, and a molten prepolymer obtained by a process comprising reacting an aromatic dihydroxy compound with a diaryl carbonate, the polymerizer apparatus including a guide-wetting fall polymerization reaction zone which has at least one guide securely held therein and extending downwardly therethrough, and allowing the polymerizable material to fall along and in contact with the at least one guide through the guide-wetting fall polymerization reaction zone, to effect a guide-wetting fall polymerization of the polymerizable material, thereby obtaining a polymer, wherein a perforated wall-surface guide is used as the guide. The perforated wall-surface guide has a plurality of through-holes, each extending substantially in the thicknesswise direction of the wall-surface guide, to thereby form openings in both surfaces of the wall-surface guide. Based on this novel finding, the present invention has been completed.
Therefore, it is an object of the present invention to provide a commercially advantageous method for producing an aromatic polycarbonate, which is free from the above-mentioned problems accompanying the conventional methods and enables an aromatic polycarbonate having a desired constant molecular weight to be stably produced at a high polymerization rate, without discoloration with respect to the polymer or generation of foreign matter.
It is another object of the present invention to provide a polymerizer apparatus for practicing the above-mentioned method.
The foregoing and other objects, features and advantages of the present invention will become apparent from the following detailed description and appended claims taken in connection with the accompanying drawings.